The focus for DWDM transport systems has, since the inception in the mid 1990's, been on increasing the bitrate per wavelength channel, the transmission distance, and reducing the cost. The transmission of 100 G (and beyond) signals over long distances over SMF has been enabled by a set of technologies like polarization-multiplexed and multi-level signalling, together with coherent detection and digital post-processing. For this reason, Dual-Pol QPSK (Quadrature Phase Shift Keying), 16-QAM (Quadrature Amplitude Modulation) and so on, are attractive solutions for high-speed transmission, both for a single carrier and multi-carrier approach (OFDM). For example, 100 G signals carried by DP-QPSK can be fit into a 50 GHz grid and it is likely that a 400 G signal carried by DP-16-QAM could be fit into a 100 GHz grid. As the spectrum of a single fibre is limited, and traffic per wavelength channel has steadily increased, the issue of spectral use has become increasingly important.
Firstly, this is due to the so-called ITU grid which divides the transmission spectrum into 100 GHz or 50 GHz slots. Thus, if a 10 Gbps channel is upgraded on a 50 GHz slot to 100 Gbps, the 100 G signal will be much more spectrally efficient per bit vs. the 10 G channel. Secondly, as the total traffic demand of the DWDM transport system increases, while new fibres are still very expensive to be deployed, the overall spectral efficiency (SE) becomes an issue.
Thus, in the last couple of years, the field of Elastic optical network has been born. The idea is to allocate as little spectrum as possible to each traffic demand. The traffic demand is simply put as an element on the network traffic matrix between two end nodes. For demands having short transmission distances and few node hops, the allocated spectrum can be made smaller since higher modulation formats can be used (more bits/symbol at the same symbol rate, i.e., more bit/s·Hz), with less effect of filter narrowing from node cascades. Also, there is a fundamental trade-off between spectral efficiency and OSNR as it is well known from communication theory. In other words, when the number of symbols used to encode the information grows and the overall power is kept constant, the average distance between the symbols in the constellation decreases and hence their tolerance to the noise.